Photocell of lead sulfide



Sept; 7, 194& R. J. cAsHMAN raorocsm. or LEAD suuxp Filed Aug. 1, 1945 Patented Sept. 7, i948 2.44am rno'roculx. or Lean sutrmr:

Robert J. Cashman. Glenview, lll., assigner to lgzlrthwestern University, a corporation of Illi- Awllcstion AUM 1. 1045, Serial No. 608,143

clalms. 1

Such cells are disclosed in my copending ap-v plications, Serial No. 521,594, filed February 9, 1944, and Serial No. 530,038, illed April '7, 1944, and the disclosures of these applications, to the extent they are applicable, are specincalb incorporated by reference into the present application.

Although photooells responsive to infra-red radiations of longer wave length have occasionally been reported to have been made, no such cell has been commercially satisfactory due to lack of stability and other causes. 'I'he present invention teaches how to produce a photocell that responds to radiations up to' 4 mu. Furthermore, the cell has other desirable characteristics such as electrical and light stability, high sensitivity, good frequency response, low resistance and impedance, high signal to noise ratio, and ease of manufacture.

These and other objects and advantages will become apparent as the disclosure proceeds and the description is read in conjunction with the accompanying drawings, in which- Fig. 1 is a, vertical sectional view showing one embodiment of the invention, the section being taken on the line I-I of Fig. 2;

Fig. 2 is a similar view but taken on the line 2-2 of Fig, 1;

Fig. 3 is a horizontal sectional view taken on the line 3-3 of Fig. l, the view being enlarged;

Fig. 4 shows a modified embodiment of the invention particularly adapted for low temperature reception, the view being a vertical section taken on the line 4-4 of Fig. 5;

Fig. 5 is a similar view but taken on the line I-l of Fig. 4, the re-entrant portion of the cell. however, being shown in elevation; and,

Fig. 6 is a horizontal sectional view taken on the line --O of Pig. 5, this view also being enlarged.

Illustrative embodiments of the invention have been shown in the drawings and preferred methods of preparing the photosensltive material and incorporating it in a cell will be described in detail, but the invention is not to be limited to these specific disclosures except as may .be required by I the prior art.

Cells responsive to wave lengths in the intermediate or far infra-red regions are particularly useful in voice and code communication systems where security is important, not only in the mes- 10 sage transmitted but also in not revealing the source of the sender. They are also useful in low temperature radiation detection, as pyrometers, and in devices for maintaining relative positions between moving aircraft and ships without havll ing the radiation source detectable by unauthorized persons.

The photosensitive material used in this invention is lead sulfide and its photosensitive properties are acquired by certain treatment of fao the material, usually as it is being incorporated ,into the cell.

Before describing the method or preparing the photosensitive material and incorporating it into a cell. it will be convenient to describe the physi- Sl cal structure of the cell as illustrated by the two types of cell shown in the drawings.

Parsrcar. Sraucrunz or Pnorocstr.

Referring first to Figs. 1 to 3, inclusive, a simple form of photocell is shown embodying the teachings of this invention. It comprises a glass envelope ID, preferably made of high melting point borosilicate glass such as Nonex or Pyrex glass, both of which readily transmit in the infrared region of the spectrum and have coefficients of linear expansion which are suited to direct sealing of the glass with tungsten electrodes Il and l! which pass through the bottom portion of the tube, the seals being indicated at i3.

Nonex and Pyrex glasses are manufactured by the Corning `Glass Works of Corning, New York, and are commercially available glasses.

The upper ends of the electrodes Il and I! are exposed within the envelope and are electrically connected by conducting lines I4 and i! to a grid II which, in the present embodiment of the invention, consists merely of extensions of the conducting lines I4 and I5 in a horizontal plane on u the interior surface of the envelope, terminating in conducting lines I1 and I8 spaced apart on the inner side Wall of the envelope. The photosensitive material I 9 is evaporated and condensed upon the inner wall of the envelope I over the grid lines I1 and I8 in a manner which will later be described.

The conducting lines I4, I5, Il and I8 may be ruled on the inner wall of the envelope before the upper end 20 of the cell is closed off and the lines may be graphite applied by lead pencil or deposited from an aqueous colloidal suspension. A material known as Aquadag manufactured by the Acheson Colloids Corporation, Port Huron, Michigan, is satisfactory for this purpose but other suitable conducting material which lends itself to application in relatively fine lines may be employed. It will be understood, of course, that the lower end of the conducting line I4 is in intimate electrical contact with the electrode II as it penetrates the envelope and that the conducting line I5 is in similar intimate electrical contact with the protruding end of the electrode I2.

The cell is evacuated through a tube 2l and when the proper pressure condition is obtained within the cell during its fabrication, the tube is sealed off as indicated at 22. The cell may be appropriately based as shown at 23.

In the form of the invention shown in Figs. 4, 5 and 6, inclusive, the construction of the cell is much the same but it is particularly designed for maintaining the photosensitlve material at a relatively low temperature, say on the order of 80 C. in order to substantially increase its -sensitivity to infra-red radiation. In this embodiment of the invention, the cell comprises an outer envelope 25 which is joined at its upper end 26 with a re-entrant portion or inner envelope 2l thereby forming an annular space 28 between theL Electrodes 29 and 30 are sealed two envelopes. through the base of "the outer envelope 25 and are connected by fine wires (which impede heat conduction with electrodes 8| and 32) anchored' in the inner envelope 21, and they are electrically connected by conducting strips 33 and 34 with' 'I'he materials used in the fabrication of thel cell shown in Figs. 4, 5 and 6, inclusive, are the same as described with respect to the embodiment of the invention shown in Figs. 1 to 3, and it should be understood that these, as well as the cell structures, may vary within the range disclosed in my prior applications, hereinbefore identified, or within the range of equivalents well known in the art. It should also be understood that under some conditions, it may be desirable to use quartz for the cell envelopes or to provide a quartz window in the portion of the envelope through which the radiations pass when falling upon the photosensitive material, because quartz transmits longer wave length radiations than Nonex or Pyrex glasses and is, therefore, particularly suitable when the cell is being used for picking up low temperature radiations.

It should also be understood that suitable filters may be used in conjunction with the cell to limit the radiations falling upon the cell to a selected band or bands of wave length.

PREPARATION or rm: LEAD SULrmr:

Photocells employing activated thallium sulfide, as described in my prior applications, aboveidentified, are difficult to make because of the care which must be exercised in handling the thallium sulfide during its preparation and activation with oxygen. The cell of this invention, by contrast, is comparatively easy to fabricate because of the less critical character of the photosensitive material; namely, lead sulfide.

The lead sulfide employed in the cell of this invention may be from natural sources such as galena crystals, or it may be prepared synthetically. When galeria is used as the source of the lead sulfide, the galena crystals are crushed to powder form (passing through a 50-mesh screen is sufficient) and commercially available grades of galena are satisfactory to use without purification. When the lead sulfide is prepared synthetically, any preparation is suitable, such, for example, as treating lead acetate or lead nitrate, which has been dissolved in distilled water, with hydrogen sulfide gas until the black lead sulfide is precipitated out. The lead sulfide is filtered out and dried. either in an oven or a vacuum desiccator. It is then fused and crushed to approprlate size and is then ready for use.

IN'rnonUcrNG THE LEAD SULFIDE INTO THE CELL After the cell has been suitably cleaned and the Aquadag or other conducting lines have been applied to the inner surface of the cell and baked, all as described in my prio-r applications hereinbefore identified, a ,few milligrams of lead sulfide, as prepared above, is placed inthe open cell and the adjacent tube wall is gently heated with an oxygen fiame to partially oxidlze the powder before it is evaporated and condensed upon the grid surface of the cell. This is done with the cell open to atmosphere and may continue for several minutes. The cell ls then connected to a vacuum pump of the conventional mechanical-oil type andla controllable air leak is provided in the vacuum line so that a pressure of 400 or 500 microns of air may be maintained within the cell during the activation process.

-While the cell is held at this pressure, the lead sulfide powder which has been loosely placed in the envelope, is shaken about so that it is directly beneath the grid surface which is to be photosensitized (the cell being in a horizontal position for the embodiments of theiinvention shown in the drawings) .and the adjacent envelope wall is then heated by a flame to cause the lead sulfide powder to sublime or evaporate and be condensed upon the grid structure, as shown in the drawings. This step must be carried on carefully because Nonex glass must be heated almost to its softening point in order to cause the lead sulfide to evaporate but with due care, the operation may be effected. Obviously, in heating-the envelope to this high temperature, the surface upon which the .deposition takes place is also at an elevated temperature, which condition contributes to the desirable operating characteristics of cation of the tube in completed.

take place. Presumably, the water vapor acts somewhat as a catalyst and it materially aids in the photosensitizing of the lead sulfide during sublimation and condensation and other vapor catalysts, characterized by having molecules or atoms possessing loosely bound electrons, i. e., molecules or atoms which have unshared electrons, would be appropriate catalysts. Mercury vapor, for example, has been found to be satisfactory.

Although lead sulfide, in its natural. state. shows slight photosensitive properties, it is only when activated with oxygen that it possesses the unusual photosensltive properties herein described. There is some reason to believe that the limited oxidation of the lead sulfide is largely responsible for this resul-t, but it is also believed that the photosensitization is partly due to the fact that, during the evaporation and condensing process, lthe lead sulfide crystal structure is changed to bring the cube faces parallel to the surface upon which the material is deposited. This is known as .preferred orientation of the crystalline layer.

Cnaasc'rnius'ncs on :ma Pnoroclu.

Spectral response and linearity of the response to light intensity Lead sulfide cells prepared in the manner described above commonly have maximum response at wave lengths on the order of .9 mu and between 2 mu and 3 mu. The response, however, of the cell extends to radiations ashigh as 4 mu and extends down to the visible spectrum.

While the response of the cell to the visible spectrum is relatively small as compared to its response to infra-red radiations, the fact that its response to light intensity is a linear function makes the cell particularly useful because it may be operated in direct sunlight without interfering with its responsiveness to infra-red radiations. It is the combination of the lack of the responsiveness to the visible spectrum and the linear character of the cell response to light intensity which gives the cell this mos-t desirable property. l

Light and electrical stability The resistance of a lead sulfide cell, prepared in the manner described, may be controlled, to a certain extent, by the amount of air left in the cell when sealed oli. It may also be controlled by the thickness of the lead sulfide layer deposited and its exposure to the catalyst, whether water vapor, mercury vapor, or some other catalytic vapor. If as much as o microns of air is left in the cell when sealed off, the resistance will drop somewhat within a few hours after the sealing of the cell takes place, or, if the cell is sealed oif at a high vacuum, the resistance generally increase for a few hours.

When the photosensitive material is subjected to low temperatures on the order of 80 C., the dark resistance of the cell may increase by a factor of 20 or more. Itis, therefore, important to construct the cell so that it has a sufficiently low initial dark resistance to prevent its cold dark resistance from becoming too high for use in amplifier systems.

The cell exhibits no fatigue due to the application of voltages of more than 100 volts D. C. and does not fatigue 'when exposed to high light intensities.

Resistance and impedance Of considerable importance is the fact that the dark resistance of the cell may be readily controlled to values which are suitable for use with amplifiers, this value, preferably, being under 10 megohms. Furthermore. lead sulfide, when activated as herein described, has relatively low resistivity which makes a simple grid structure possible. It also has relatively low impedance which favorably affects the signal to noise characteristics of the cell.

Signal to noise ratio In any photocell, the signal to noise ratio is an important factor in determining the usefulness of rthe cell, particularly in voice communication systems. Using the same -tungsten source at 2870 Kelvin and" a 110 cycle modulation of the source, and for comparable areas of exposure, the lead sulfide cell of this invention has a signal to noise ratio which is possibly 15 decibels below that of the thallium sulfide cell disclosed in my prior applications but possibly 20 decibels higher than the signal to noise ratio of an ordinary gas filled phototube of the caesium type.

However, the cell shows a tremendous increase in sensitivity at low temperatures, the cells responsiveness peaking at around 80 C'. which, fortunately, is near the temperature of solid carbon dioxide. When the photosensltive material shown at 3l, Figs. 4, 5 and 6, is cooled to Dry Ice temperature by packing the re-entrant portion of the `tube with solid carbon dioxide, as shown at I0, the cell sensitivity is increased by a factor oflapproximately 500 with the noise level remaining about the same.

Ffequency TeSpOTlSe A lead sulfide photocell made in accordance with this invention is particularly suited for voice communication systems because of its favorable frequency response. The cell exhibits a drop of only 41/2 decibels up to 20,000 cycles and this is substantially better than thallium sulfide cells which are relatively unresponsive to frequencies over 6000 or '1000 cycles. The favorable frequency response of the lead sulfide cell also adapts itself to many other purposes where quick yresponse to change in radiation is important, as, for example, scanning systems for low temperature radiation sources.

There is some evidence that in the illustrative procedures for activating lead sulfide, a part of the oxygen treatment is strictly a chemical oxidation process producing some lead Oxy-sulfide and a part is more or less of an absorption of oxygen by the lead sulfide. This probably takes place in any successful activation of lead sulfide. For example, I have found that I can take lead sulfide powder and lead oxide powder, fuse them together in -a closed vessel, crush the resultant mass, and then evaporate and condense upon the grid structure of a cell. as hereinbefore described, and obtain good results.

Therefore, in the appended claims, the term "lead sulfide is not to be construed in its strict chemical sense but should be regarded as including material which may vary somewhat from chemically pure lead sulfide. From this standpoint, lead sulfide, as used in the specification and claims may be regarded as a lead-sulphur solution in which there may be a slight excess of either lead or sulphur, and when activated, may be regarded as including oxygen,

either chemically combined therewith, or in some other manner associated therewith.

The expression in the appended claims that the photosensitive material is formed as a de- DOsit in the cell under controlled oxidation cond-itions and maintained in its activated state by not thereafter subjecting it to uncontrolled oxidation or words of similar import is intended to mean that the photosensitive material is in the form of a deposit in the cell; that itis activated either during or after the forming of the deposit; and that during the deposition and thereafter the material is under controlled oxidation conditions.

I claim: f

1. The method of making a photoconductive resistance element which consists in depositing lead sulfldeupon a surface -by evaporation and condensation, and subjecting the lead sulfide during or after deposition to limited quantities of oxygen.

2. The method of making a photoconductive resistance element which consists in depositing lead sulfide upon a surface by evaporation and condensation, and subjecting the lead sulfide during or after deposition to limited quantities of oxygen in the presence of a catalyst.

3. The method of making a photoconductive resistance element which consists in taking a quantity of lead sulfide and causing it to have a limited reaction with oxygen in the presence of a catalyst and while under the influence of heat, the catalyst being chosen from a group whose molecules or atoms are characterized by having unshared electrons.

4. The method of making a photoconductive resistance element comprisingactivated lead sulfide, which consists in forming a deposit of lead sulfide on a surface and causing it to have a limited rea-ction with oxygen while maintained under at least a partial vacuum in the presence of a catalyst and while under the influence of heat, and Ithen sealing the activated material from the atmosphere.

5. The method of making a photoconductive resistance element comprising activated lead sulfide which consists in forming a deposit of lead suliide on a surface and subjecting it to a limited quantity of moist oxygen while under the influence of heat, and then sealing the activated material from the atmosphere.

6. The method of making a photoconductive resistance element which consists in taking lead sulfide, evaporating and condensing it upon a surface while maintaining the surface at an elevated temperature, activating the lead sulfide with oxygen either during or subsequent to said evaporating and condensing step, and then sealing it from the atmosphere.

7. The method of processing a lead sulfide photocell which comprises the steps of forming a deposit of lead sulfide on a surface within the cell while maintaining the cell under at least a partial vacuum, and causing limited oxidation of the lead sulfide to take place under the influence of heat during or after the formation of said deposit.

8. The method of preparing a photocell having an envelope enclosing a photosensitive element which consists in depositing a thin layer of lead sulfide on a surface within the envelope by evaporating a quantity thereof in the envelope while under at least a partial vacuum, and causing it to condense upon said surface, activating the lead sulfide by admitting limited quantities of oxygen to the envelope while the lead sulfide is at an elevated temperaturev,.and then sealing the envelope from the atmosphere.

9. The method of preparing a photocell having an envelope enclosing a photosensitive element which consists in forming a deposit of lead sulfide on a surface Within the envelope whilev the envelope is maintained under partial vacuum and While admitting to the envelope limited quantities of oxygen, then removing all excess oxygen from the envelope and sealing the envelope.

10. The method of making a photocell having a photosensitive element hermetically sealed therein which consists in forming a grid of conducting lines on a surface within the envelope, then while the envelope is under at least a partial vacuum depositing lead sulfide on the grid by causing a quantity thereof to be condensed on said grid, admitting limited quantities of oxygen to the envelope While subjecting the lead sulfide to the application of heat, and finally hermetically sealing the envelope from the atmosphere.

11. The method of making a photoconductive photocell which consists in placing a quantity of a photosensitive material in an enclosure, de-

l positing the photosensitive material on a heated surface within the enclosure while maintaining the enclosure at a relatively low pressure, admitting limited quantities of oxygen to the enclosure to thereby activate the photosensitive material, and then hermetically sealing o' the enclosure without subjecting the deposited material to uncontrolled oxidation.

12. The method of making a photoconductive resistance element which comprises taking lead sulfide and depositing it on a heated surface to form a preferred orientation of the crystalline structure thereof 13. The method of making a photoconductive resistance element which comprises taking lead suliide, depositing it on a heated surface to form a preferred orientation of the crystalline structure thereof, and subjecting the lead sulfide to a limited oxidization process either during or after its deposition on said surface 14. The method of making a photoconductive resistance element which consists in causing lead sulfide and oxygen to react under the influence of heat and to be deposited upon a heated surface, and then hermetically sealing the surface.

15. The method of making a photoconductive resistance element which consists in causing lead, sulphur and oxygen to have a limited reaction, then depositing the resultant material upon a heated surface, and either during or after such deposition further causing said lead, sulphur and oxygen to react, and finally herm'etically sealing the surface with the material deposited thereon.

16. The method of making a photocell which comprises taking an envelope, heating the envelope, evaporating lead sulfide in the envelope to cause it to be deposited upon a heated surface within the envelope while maintaining the envelope at a relatively low pressure and while bleeding into the envelope a quantity of air or oxygen, and then sealing off the envelope.

17. The method of making a photoconductive resistance element which consists in taking lead sulfide and lead oxide, evaporating and condensing the lead sulfide and lead oxide upon a surface, and then sealing the surface from the atmosphere.

18. The method of making a photoconductive resistance element which consists in taking lead sulfide and lead oxide, fusing them together,

the crystalline layer substantially parallel to the surface. y

21. -A photoconductive resistance element comprising lead sulfide which has been activated with oxygen in the presence of a catalyst `and while under the influence of heat, the lcatalyst being chosen from a group whose molecules or atoms are characterized by having unshared electrons.

22. A photoconductive resistance element comprising lead sulfide activated with oxygen while under the influence of heat, and at reduced pressure, and characterized by its responsiveness to infra red radiation up to approximately 4 mu, and by its substantially greater sensitivity when maintained at a temperature on the order of 80 C.

23. A photcconductive resistance element comprising lead sulfide and lead oxide which have been fused together and activated with oxygen.

24. A photoconductive resistance element characterized by its photosensitiveness to infra red radiations and composed of lead sulfide and oxygen-containing compounds of lead formed under controlled oxidation conditions and with the application of heat, and maintained in stable form by being hermetically sealed from the atmosphere.

25. A resistance element composed of lead, sulphur and oxygen reacted with the application of head and under controlled conditions to form a photosensitive material after which said photosensitive material is kept in stable form by being hermetically sealed within an envelope.

26. In a photosensitive cell, a hermetically sealed envelope having a wall capable of transmitting radiations in the infra red region of the spectrum, a photosensitive surface in the envelope adapted to receive infra red radiations passing through said wall, said surface including a deposit of activated lead sulfide which has been formed on the surface by evaporating a quantity of lead sulfide in the envelope while under at least a partial vacuum in an oxidizing atmosphere but after the lead sulfide has already been partially oxidized.

27. A photosensitive cell of the type including a photosensitive element encased within a hermetically sealed container, said cell being characterized by a dark resistance of less than 50 megohms, high sensitivity to radiations between 1.5 mu and 4 mu, and a noise factor approaching that produced by thermal effects alone, said cell including a photosensitive surface of lead sulfide activated with oxygen under the influence of heat and deposited on a grid formed of graphite deposited from a colloidal graphite suspension.

28. In a photoconductice cell, a sealed envelope, a photosensitive surface in the envelope formed as a deposit in the cell under controlled oxidation conditions and maintained in an activated state by not thereafter subjecting it to uncontrolled oxidation, a pair of electrodes projecting into the envelope, and conductors including relatively fine grid lines connecting the electrodes to spaced portions of the photosensitive surface, said grid lines constituting a deposit of graphite from a colloidal graphite suspension.

29. A photosensitive cell comprising a hermeticaliy sealed envelope having a photosensitive material deposited on the inside surface thereof, which material is activated under controlled oxidation conditions and maintained in its activated state by not thereafter subjecting it to uncontrolled oxidation, electrodes sealed through the envelope, means for connecting the electrodes to laterally spaced portions oi.' the photosensitive material including a conducting path formed on an inside surface of the envelope such that conduction inthe photosensitive material occurs in a direction parallel to its surface.

30.' A photosensitive cell having a photoconductive element which includes a grid of relatively fine conducting lines formed by depositing small graphitic particles from a liquid, and a photosensitive material deposited over the grid and activated under controlled oxidation conditions' tained in its activated state by not thereafter su-bjecting it to uncontrolled oxidation.

32. In a photosensitive cell, a hermetically sealed glass envelope having a re-entrant portion closed at its inner end, a photosensitive material deposited on an interior surface of said reentrant portion with a grid associated therewith composed oi spaced parallel conducting lines, a pair of conductors sealed through the envelope, one of which is connected to one of said spaced parallel conducting lines and the other conductor being connected to the other of said conducting lines.

33. In a photosensitive cell, an hermetically sealed glass envelope having a pair of electrodes sealed through the base thereof and having a reentrant portion closed at its inner end, a photosensitive material deposited on an interior surface of said re-entrant portion with a grid associated therewith composed of spaced parallel conducting lines, one of which is electrically connected to one of said electrodes and the other being connected to the other of said electrodes, said reentrant portion ofthe envelope forming a container for the reception of cooling means for lowering the temperature of said photosensitive material and thereby increasing its sensitive'ness.

34. A photosensitive cell comprising an hermetically sealed envelope having a photosensitive material deposited on an inside surface thereof, electrodes sealed through the envelope, means for connecting the electrodes to laterally spaced portions of the photosensitive material such that conduction in the photosensitive material occurs in a direction parallel to its surface, said photosensitive material comprising lead sulfide and oxygen reacted under the influence of heat.

35. A photosensitive cell comprising an hermetically sealed envelope having a photosensi- 11 n 12 tive material deposited on an inside suriace UNITED STATES P mn. thereof, electrodes sealed through the envelope, A s means for connecting the electrodes to laterally Number Namo Date spaced portions oi the photosensitive material 420,884 Mercadier Feb. 4. 1890 such that conduction in the photosensitive m8'- 5 `'155.840 Bose Mar. 29. 1904 terial occurs in a direction parallel to its sur- 1,316,350 Case sept. 1e, 1919 face, said photosensitive material comprising 1,419,040 Hart Apr. 22, 1924 lead, sulphur and oxygen reacted under thein- 1,601,607 Wein sept, 2a, 192s fiuence or heat, and the inner surface upon which y y 1,807,056 Zworykin May 26, 1931 the photosensitive material is deposited being an 10 2,034,334 Falkenthal Mar. 17, 1936 inner wall of the envelope itself. FOREIGN PATENTS ROBERT J. cAsHMAN. Number Country `Date 271,401 Great Britain June 16, 1927 REFERENCES CITED l5 l f OTHER REFERENCES Allen, Photo-Electricity, 1918, pages 'I5-77. The followintg rteierences are oi' record in the Hughes et Il Phqtlecmc Phenomena' 1932, ille oi this pa en pages 37N.

Certificate of Correction Patent No. 2,448,516. September 7, 1948.

vROBERT J. CASHMAN It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 5, line 63, for the Word increase read increases; column 9, line 44, `claim 25, for head read heat; line 71, claim 28, for photoconductic'e read photoconductive; I

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent OfIce.

' Signed and sealed this 11th day of January, A. D. 1949.

THOMAS F. MURPHY,

Assistant Uommzseoner of Patents. 

